Last game I had 8 (well, 9 if you count the one I founded from a convert that I got on my last turn of the game). 3 were conquered ones. If I were actually building up a full economy in the game, I could easily have used 4 more colonies: 1 to get furs, 1 to get sugar, 1 to convert furs to coats, 1 to convert sugar to rum. Maybe another one as just a lumber pump city to produce like crazy. With my 8, I probably had about 2 too many. 6 seems about perfect:
1 tools/guns town
2 factory towns to pump out cigars/cloth/rum/coats (depending what resources you harvest)
1 food town
2 raw materials towns, which convert to food towns when you declare independance.